FEATURE: Publish read state on group messages. (#7989)

* Enable or disable read state based on group attribute

* When read state needs to be published, the minimum unread count is calculated in the topic query. This way, we can know if someone reads the last post

* The option can be enabled/disabled from the UI

* The read indicator will live-updated using message bus

* Show read indicator on every post

* The read indicator now shows read count and can be expanded to see user avatars

* Read count gets updated everytime someone reads a message

* Simplify topic-list read indicator logic

* Unsubscribe from message bus on willDestroyElement, removed unnecesarry values from post-menu, and added a comment to explain where does minimum_unread_count comes from

spec/requests/post_readers_controller_spec.rb

@@ -0,0 +1,90 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+describe PostReadersController do
+  describe '#index' do
+    fab!(:admin) { Fabricate(:admin) }
+    fab!(:reader) { Fabricate(:user) }
+
+    before { sign_in(admin) }
+
+    before do
+      @group = Fabricate(:group)
+      @group_message = Fabricate(:private_message_topic, allowed_groups: [@group])
+      @post = Fabricate(:post, topic: @group_message, post_number: 3)
+    end
+
+    context 'When the user has access to readers data' do
+      before do
+        @group.update!(publish_read_state: true)
+        @group.add(admin)
+        @group.add(reader)
+      end
+
+      it 'returns an empty list when nobody has read the topic' do
+        get '/post_readers.json', params: { id: @post.id }
+
+        readers = JSON.parse(response.body)['post_readers']
+
+        expect(readers).to be_empty
+      end
+
+      it 'returns an user who read until that post' do
+        TopicUser.create!(user: reader, topic: @group_message, last_read_post_number: 3)
+
+        get '/post_readers.json', params: { id: @post.id }
+        reader_data = JSON.parse(response.body)['post_readers'].first
+
+        assert_reader_is_correctly_serialized(reader_data, reader, @post)
+      end
+
+      it 'returns an user who read pass that post' do
+        TopicUser.create!(user: reader, topic: @group_message, last_read_post_number: 4)
+
+        get '/post_readers.json', params: { id: @post.id }
+        reader_data = JSON.parse(response.body)['post_readers'].first
+
+        assert_reader_is_correctly_serialized(reader_data, reader, @post)
+      end
+
+      it 'return an empty list when nodobody read unti that post' do
+        TopicUser.create!(user: reader, topic: @group_message, last_read_post_number: 1)
+
+        get '/post_readers.json', params: { id: @post.id }
+        readers = JSON.parse(response.body)['post_readers']
+
+        expect(readers).to be_empty
+      end
+
+      it "doesn't include current_user in the readers list" do
+        TopicUser.create!(user: admin, topic: @group_message, last_read_post_number: 3)
+
+         get '/post_readers.json', params: { id: @post.id }
+         reader = JSON.parse(response.body)['post_readers'].detect { |r| r['username'] == admin.username }
+
+         expect(reader).to be_nil
+      end
+    end
+
+    def assert_reader_is_correctly_serialized(reader_data, reader, post)
+      expect(reader_data['avatar_template']).to eq reader.avatar_template
+      expect(reader_data['username']).to eq reader.username
+      expect(reader_data['username_lower']).to eq reader.username_lower
+    end
+
+    it 'returns forbidden if no group has publish_read_state enabled' do
+      get '/post_readers.json', params: { id: @post.id }
+
+      expect(response).to be_forbidden
+    end
+
+    it 'returns forbidden if current_user is not a member of a group with publish_read_state enabled' do
+      @group.update!(publish_read_state: true)
+
+      get '/post_readers.json', params: { id: @post.id }
+
+      expect(response).to be_forbidden
+    end
+  end
+end